Process for the production and refining of metals



Patented May 17, 1949 E PRODUCTION AND rnocnss FOR 'rn REFINING or METALS Philipp Gross, Slough, England, asslgnor to International Alloys Limited, Aylesbury, England No Drawing. Application July 22, 1947, Serial No. 762,827. In Great Britain March 27, 1946 Section 1, Public Law 690, August 8, 1946 Patent expires March 27, 1966 This invention relates to improvements in the distillation of metals and is a continuation-inpart of my copending application, Serial No. 589,093, filed April 18, 1945.

It is well known in the art that metals such as mercury, arsenic, cadmium,, zinc, magnesium, thallum, and the alkal metals, which have an appreciable vapour pressure at technically practicable temperatures, can be refined and/ or produced by direct distillation, i. e. by transferring the metal into the vapour phase by heating material containing the'metal to elevated temperatures under normal pressure or, in order to reduce the distillation temperature, under reduced pressure.

This invention provides a distillation process for producing or refining metals which have not a sufiiciently high vapour pressure at technically practicable temperatures for direct distillation, hereinafter simply referred to as normally nonvolatile metals. In this connection the expression metal" is to be understood as also including transition elements between metals and metalloids such as boron. Accordingly the expression "normally non-volatile metals" hereinafter refers to these metals and transition elements.

The distillation process of the invention can be applied to impure normally non-volatile metals, to their alloys or to inter-metallic compounds containing one or more normally non-volatile metals, to their carbides, nitrides, or similar compounds, and in the presence of a suitable reducing agent such as carbon to their oxides or compounds derived from those oxides. The term distil" (distillation) as used in this connection thus comprises purifying (purification) and producing (production) by distillation normally nonvolatile metals from materials bearing the Same. The term metal bearing material thus includes the impure metal as well as any of the mentioned substances or mixture of substances.

The invention is based on the existence in the vapour phase of such lower halides of normally non-volatile metals as for instance A131, A101, Alli, BBr, BCl, BF, CoBr, CoCl, FeCl, GeBr, GeCl, MnF, NiBr, NiCl, SnBr, SnCl, SnF, TiCl, in which the. metal has a lower valency than in its stable halides, which therefore are not stable at room temperature and the existence of which in the vapour phase has been ascertained by such indirect means only as their absorption spectra.

According to the invention, metals are transferred into the vapour phase at temperatures below their temperatures of direct evaporation by approaching or establishing. equilibrium be- 11 Claims.

. 2 tween the solid or liquid metal bearing material and the vapour of a higher halide of the metal whereby the vapour of a lower halide is formed within the resulting vapour mixture.

By the expression "a higher metal halide a halide of the metal is to be understood in which the metal has a valency of more than one and which can be brought to reaction temperature without decomposition into metal (and halogen). If more than one such halide exists that stable halide in which the metal has the lowest valency is preferably used.

The metal can be recovered from the resulting vapours for instance by converting them by cooling into the metal and the original or other stable halide or by absorption of the metal. If, however, the metal bearing material gives rise to the formation of potentially oxidising vapours or gases, such as carbon monoxide in the case of the mixture of carbon and metal oxide being the metal bearing material, special precautions might have to be adopted, which are well known in the art from the carbothermic reduction of magnesia; for instance shock cooling (chilling) of the reaction vapours.

Under a modification of the invention the higher halide is replaced by the mono or higher halide of a metal other than the metal to be distilled which metal is volatile under the conditions of reaction whereby the resulting vapour mixture contains the vapour of this other metal together with the lower (and higher) halide of the metal to be distilled. If metal bearin material is used which does not form potentially oxidising vapours the metal distilled and the halide of the other metal are recovered from the reaction vapours by gradually cooling them.

Under another modification of the invention use can be made of halogen containing substances, such as the hydrogen halides or the halogens themselves, which react in a not completely reversible way with the 'metal bearing material forming a mixture of lower and higher halide. In this case the gradual cooling of the vapours of the reaction leads to the condensation of the metal and of its higher halide if the metal bearing material does not give rise to potentially oxidising vapours or gases.

' The lower halide is in many cases a monohalide and, of course, must be if the higher halide is a bihalide. If the higher halide is a halide of the metal to be distilled and if the lower halide is a monohalide, the equation is:

(n-l) Mewim+MexnzinMexvspour (I) 3. in which Mesolid denotes the metal to be distilled in its solid or liquid state, X the halogen, Mex" the higher halide and Mexvapour the lower halide vapour. For instance, if aluminium is the metal to be distilled and aluminiumtrichloride is used as higher halide the equation becomes:

zAlsolid+Alcl3vapouri 3Alclvapour 'If the lower halide is a monohalide and if the monohalide of another volatile metal is used the reaction takes place according to the equation:

Mesolid+MaXvapourZMeXvapour+Mavapour in which the same denotations are used as in (I) and in which Maxvapour and Mavapour mean the vapours of the other halide and the other metal itself respectively. For instance if aluminium is again .the metal to be distilled and sodium chloride is used instead of the higher halide, the reaction is represented by the equation:

Alsolid+NaC1vapour2A1C1vapour+Navapour Among the halides, chlorides and fluorides are preferred though bromides and in some cases iodides have been found effective.

In order that a metal with too low a vapour pressure for direct distillation can advantageously be distilled according to the present invention the following conditions must be fulfilled:

(1) The monohalide of the metal must not be stable at temperatures essentially below the reaction temperature (it may or may not become unstable at temperatures essentially above reaction temperature) A halide (higher halide) must exist in which the metal has a valency of more than one and the vapour of which under the conditions of reaction does not appreciably dissociate into metal (and halogen);

halogen-atom of the higher halide vapour in which the metal has the lowest valency of all stable halides divided by the valency of the metal in this higher halide must be smaller than the heat of dissociation of the lower (unstable) halide vapour into metaland halogen-atom divided by the valency of the metal in the lower halide; and

If under modification of the invention the halide of a metal other than the one to be distilled is used, which metal must be volatile, the average heat of dissociation of this halide vapour into metaland halogen-atom must be smaller than the corresponding average heat of dissociation of the unstable lower halide.

The invention provides a method for obtaining a normally non-volatile metal in the metallic state by a distillation process which comprises reacting the metal bearing material at elevated temperatures, below the temperature of direct evaporation of the metal from the said material (preferably under partial vacuum) with a reacting halogen containing substance brought to the vapour phase previous to reaction, thereby by a strongly endothermic reaction to form the vapour of a lower halide of the metal and recovering the metal from the resulting unstable lower halide vapour by condensation brought about by cooling or absorption or a combination of these two I measures.

The heat of dissociation into metaland distilled or the halide of another and volatile metal, which halides (reacting halides) obey the aforesaid conditions (2), (3) or (4), respectively, or which is another halogen containing substance known to be capable of reacting with the metal to form its higher halide (reacting halide), also obeying the aforesaid conditions.

The method is carried out for instance by conducting the vapour of the reacting halogen containing substance over the solid or liquid metal, which is preferably broughtto a condition where it oiTers a high specific surface area to the reacting halogen containing substance at elevated temperatures and preferably under reduced pressure, and cooling the resulting vapours in one or more suitable condensers whereby they decompose to the metal and the original halide.

The best conditions for carrying out the distillation can be found from the following equilibrium considerations which for the sake of simplification have been formulated for a mono-.

halide as lower halide. By passing the vapour of the higher metal halide over the metal, equilibrium between higher and lower metalhalide will be substantially established according to Equation I and the equilibrium constant is given by p MeX pMIX where K denotes the equilibrium constant which depends on the temperature only, and pmx and pMeX denote the partial pressures of lower and higher halides respectively. For instance, for aluminium distilled in an atmosphere of aluminiumtrichloride this equation is If m denotes the pressure with which the higher halide enters the reaction chamber and a denotes the fraction of higher halide actually converted into lower halide, thus measuring the efficiency of the reaction, the equation for the equilibrium constant can be written as Kz n n rr-l 1-11 in the case of the example This equation shows that the reaction will be the more complete the lower the initial pressure or the initial partial pressure of the higher halide. However, the pressure should not be too low because the actual amount distilling per unit of time and unit of surface area of the metal bearing material would then be too low. .Pressures of about 0.1 mm. mercury appear to be a limit for large scale production.

The chemical Reaction I is endothermic and metal and halogen in which, again for the sake of simplification, the lower halide is assumed to be a monohalide.

The generalized thermo-chemical reaction for the evaporation of a metal can be written:

In this equation Mesol and Mevapour indicate the metal in its solid (or liquid) state and in its vapour state respectively, and L1 is the heat of transference into the gaseous state-in this case simply the heat of evaporation. For instance, in the case of aluminium at 1000 C.

A1liquid=A1vapour-72,63O cal.

because S1, the entropy of evaporation under standard pressure, does not vary much for the various metals. (Most metals obey quite well 'Troutons rule, according to which the entropy change at the boiling point is a general constant.) In the case of aluminium at about 1000 C. again this formula becomes in which the values of 39-31 and 6-75 for the entropies of gaseous and solid aluminium at 25 C. respectively, the above molar heats, and heat of fusion of aluminium have been used.

The concentration in the gaseous phase, i. e. the apparent vapour pressure for all distillation purposes will be raised and distillation facilitated by any means by which the heat of transference into the gaseous state can be reduced and which has not an equal or greater opposite effect on the entropy of transference into the gaseous state. If S1 remains unchanged any reduction in L1 will effect an essentially proportional reduction in the practical distillation temperature, measured on the absolute temperature scale.

The thermochemical reaction of the dissociation of an unstable lower and a stable higher halide vapour into gaseous metaland halogenatoms are:

MexvapouraMe vapour-i-xatom--D1 (III) and.

MBXnvapourPMBvn-pourd-nXatom-7l5 (IV) that is, the thermochemical equation for the distillation Reaction I. By

dividing by (n1) and so making the reaction comparable with the Equation II for direct distillation one gets:

|cl+ :i nvlpour 1 Tg'i vapuur This equation represents the thermochemical reaction for the transference of one gramme atom of metal into the vapor phase according to the invention. By comparison with (II) it is seen that the heat for transference of the metal is changed by i (Di-1 Dl-5 (at room temperature) becomes 18,000 cal. and the expression equals 27,000 cal, thus bringing about a very considerable reduction of the value of L1'74,600 (for liquid aluminium) to For the calculation of the value for D15' use has been made of the lower limit (118,000) of the band spectroscopically determined value of 123,100:4,610 cal. for D1, and '15 has been 0211-- culated from the heat of formation of crystallised aluminiumtrichloride (Q=167,900 cal), its heat of evaporation (28,850 cals.) and the heat of dissociation of the dimerous molecule into monomerous vapour (28,800 cal.) (L2=28,825 cal.) using the value D2=56,900 cal. for the heat of dissociation of chlorine molecules into free atoms. By an exactly analogous calculation it can be shown that in general where the lower halide is not a monohalide the condition is quite analogous, namely Dm D where mm is the heat of dissociation of the gaseous lower halide MeXm into the metal atom and its m halogen atoms. If the difference (DmD) is positive the formation of the lower halide vapour from the vapours of the higher halide and the metal according to:

7LMeXmvapour+mn(1 m-E) is exothermic, and the distillation reaction ac-' cording to the invention is less endothermic than the direct evaporation, which means that it takes place at lower temperatures. Such a difference in the average dissociation energies of lower and higher halide can be expected in the halides of those metals in which, as apparent from the spectra of the metal-atoms and ions, the electrons of the metal atom performing the bond in the higher halide are not all energetically equivalent, more especially if they are not all of the same type; for instance, in aluminiumhalides the three electrons perfecting the bond are one 3p and two 3s electrons. This difierence must, however, not be so great as to make the lower halide stable at room temperature.

Exactly similar calculations can be made if the I tively; the value of 72-8 h s 4.575T 4.575 In this equation it means the heat of the reaction in calories at the absolute temperature T,

and s the standard entropy of reaction at that temperature. The heat of reaction is given by:

in which mDm means the heat of dissociation of the aseous lower halide into metaland halide-atoms, D2 the heat of dissociation of the gaseous halogen molecule X2 into its atoms 2X which is well known for all the halogens, L1 and L: the heat of evaporation of the metal and the higher metal-halide respectively and Q the heat of formation of the solid higher metal-halide from the solid metal and gaseous halogen. The

log K= 'he at of dissociation of the lower metal-halide mDm has been determinedaccurately enough for a great number of such halides especially monohalides from their band spectra, failing which it must be estimated by the rules of analogy and interpolation, having due regard for the ionization energies of the metal atom into its various (single, double and so on charged) ions and their ground state.

Using the thermochemical values as before and the values of 9 and for the molar heats of aluminium monochloride and aluminium trichloride respectively, the reaction heat h in the case of aluminium trichlori-de and aluminium to form aluminium monochloride at 1000 C. amounts to 89,300 cal.

The entropy of reaction under standard pressure s is given by:

mvanour (n m) sul m'auour in which SMe Szvnx and SMeX denote the entropies under standard pressure of the solid or liquid metal and the gaseous higher and lower halide respectively. These entropies are either known through direct measurements, as for most of the metals, or can be calculated, as for many of the halide vapours, or they can be estimated accurately enough by well known rules.

The thermo-dynamical magnitudes h and s have to be taken at, or corrected to the reaction temperature by the relevant laws and rules.

For instance, the entropy of solid aluminium at C. has been measured (SA1=6.75 the entropy of aluminium monochloride at 25 C. can be' exactly calculated from band-spectroscopic values (Smc1=53.73) and the entropy of aluminium trichloride vapour can either be interpolated from the known entropies of similar molecules of the formula AX: or the molecular constants can be estimated and the entropy calculated; the values are 71 -4 and 73-1 respechas been adopted.

(The

' mospheric or elevated If the initial aluminium trichloride pressure is for instance 3 mm. mercury i. e.

3 ate. a is given by or a=0.94, in good agreement with experiments described in Example I.

Perfectly analogous equations determine the reaction temperature when the vapour of the halide of a volatile metal is used as reacting halide vapour.

As already mentioned the fluorides and chlorides are preferred, though bromides may be quite effective. Iodides are usually less effective because the equilibria at higher temperatures are adversely affected by the generally much lower bond energies between metal-and iodine atom, and between iodine atoms themselves, which leads to free iodine atoms in the equilibrium mixture thereby reducing or suppressing the reaction eifective for metal volatilization.

When practising the invention the reacting halide vapour, i. e. the vapour either of a higher halide of the metal to be distilled or of a halide of another suitable volatile metal is brought into contact at elevated temperatures with the metal bearing-material preferably under reduced pressure or as admixture to an inert gas itself under normal, subnormal or slightly supernormal pressure. For that purpose the metal bearing material is brought in a condition in which it offers a. high specific surface area to the halide vapour. If it is solid at the reaction temperature it is preferably used in the form of a coarse powder, loose or in the form of porous briquettes; if liquid, it is spread as a film or in the form of drops over non-reactive material of a high specific surface area, or it may be dispersed as a. spray within the higher halide vapour or its mixture with an inert gas.

The reacting halide vapour may be introduced into the reaction chamber as such, preferably under reduced pressure, or contained in some indifferent carrier gas under reduced, at-

pressure. The reacting halide vapour may also be generated in the reaction chamber by introducing into it and placing at a spot of appropriate temperature a solid (or liquid) substance from which higher halide vapours originate on heating, e. g. the solid or liquid higher halide or a compound (for instance a complex halide) which dissociates into the reacting vapour or vapours and into solids or liquids with no appreciable vapour pressure or into inert or harmless vapours. For instance, in the case of boron as metal to be distilled and borontrifiuoride as higher halide, this gas may be generated in the reaction chamber by intro- 9 ducing solid sodium or potassium-borofluoride into the chamber.

The reacting halide in equilibrium with the lower halide may also be generated in the chamber by introducing into it such substances as halogen (such as chlorine) or hydrogen halide (such as hydrogen chloride) which lead to the formation of higher halide by reaction with the metal-bearing material. For instance, boron can be distilled by the reaction between boron carbide and chlorine to form the equilibrium mixture of boron-monochloride and trichloride.

Depending on the nature of the halogen containing substance used and the metal bearing material and the procedure of distillation, the halide condenses either entirely separated from the metal or to a varying degree together with it. A necessary condition for separate condensation is that the vapour of the reacting halide under the conditions of contact with the metal bearing material is in an unsaturated state, i. e. of a pressure or partial pressure which is lower than the vapour pressure of the halide at the temperature of contact. This can be achieved for instance by keeping the pressure in the system practically constant and the reaction temperature above the temperature of the evaporation of the halide, or by allowing for expansion or dilution of the halide vapour from the place of evaporation to the place of reaction at prac tically equal temperature of these two places, or by combining these two measures. If the partial pressure of the reacting halide under the appropriate conditions of reaction is small in comparison with its vapour pressure at reaction temperature (highly unsaturated vapour) the metal condenses at much higher temperatures than the halide and therefore entirely separated from it. It is therefore advantageous to use such halides for reaction which, under the pressure existing in the system, sublime or boil at tem peratures far below reaction temperatures (low boiling or subliming halides). In this case the halide is afterwards either condensed at much lower temperatures or re-circulated into the reaction system without condensation at all by keeping the metal condenser and all other parts of the system including the circulation pump above the condensation temperature of the ha1- ide. least two halide condensers are employed and utilised alternately as halide condenser and as halide evaporator whereby the same amount of halide is repeatedly used for reaction with the metal bearing material essentially without dish continuing the distillation. Separate condensation takes place for instance when aluminum is distilled in a stream of aluminiumtrichloride; circulation can easily be arranged if boron is distilled in a stream of borontrichloride. If on the other hand the vapour pressure of the reacting halide at reaction temperature is comparable with, though greater than (unsaturated vapour nearer to saturation), its pressure in the reaction mixture, the metal condenses partly together with' the higher halide and has to be separated from it by mechanical or other means, for instance, melting and segregating. It is therefore most advantageous to choose the reacting halide, its pressure and the reaction con ditions so that the temperature of condensation of the halide is low in comparison with the temperature of efiective reaction. If the condensed halide contains only relatively little of the distilled metal it can be reused without further If the halide is condensed preferably at 10 separation, without danger of reducing the subsequent yield or yields appreciably.

The metal may finally be recovered from the vapour by absorption through a suitable absorption medium (for instance, a'nother liquid or solid metal) from which it can afterwards be separated.

Substantially by the same process one metal or group of metals A may be freed from another metal or. group of metals B with which it is mixed or alloyed by subjecting the mixture to the distillation process according to the present invention under conditions suitable for the dis tillation of the metal or metals B and retaining thereby the metal or metals A as a distillation residue practically free from the metal or metals B.

' Example I A tower filled with re-crystallised high purity alumina was used as reaction chamber, aluminium being kept as liquid. or molten droplets on the alumina and moving slowly from the top to the bottom. The tower was brought up to the desired temperatures by electric currents induced in a shell of low carbon iron. The bottom end of the tower was connected with an halide evaporator containing the halide to be used, and electrically heated, and its top end was connected with a condenser, water-cooled at its top end and connected with a vacuum pump. The aluminium usually condenses in a zone near the reaction chamber at a temperature estimated to be about 700 0., whereas the halide used condenses in the water-cooled zone of the condenser. The residual pressure was in every case less than 0.5 mm. mercury and in most casesconsiderably less. The time of distillation was between one and four hours.

In a series of experiments made in this apparatus, the aluminium bearing material was impure aluminium containing Cu 3.36%, Ni 0.75%, Fe 1.28%, Si 1.74%, Mn 0.35%; but the impurities in the distillate averaged, Cu less than 0.05%, Fe less than 0.06%, Si less than 0.04%. Manganese trace to 0.05%. The halide used was aluminium chloride evaporated at about 120 C. When the temperature of the reaction tower was kept at 900 to l,000 C. the ratio of the weight of aluminium chloride having passed through the aluminium bearing material to the weight of aluminium distilled was about 2.5 to 2.6 rising to about 2.8 to 3.0 when the reaction temperature was lowered to about 800 C. and becoming more unfavourable to values between 4 and 7 when the reaction temperature was reduced to about 700 C. or when the evaporation temperature of the aluminium chloride was raised to 130 C. or

more.

With AlBrz in the same apparatus aluminium was distilled with the halide evaporator main tained at about 100 C. and the reaction tower at about 1,000 C.

Example II of the weight of the aluminium chloride to the 75 weight of the aluminium distilled averaged 2.7

at reaction temperatures round about 930 C. and rose to values of about 7 and higher at temperatures of 830 C. and lower.

Example III In a third series of experiments, ferro-aluminium of various compositions was used. These materials werebrought into the reaction chamber as a coarse powder, no supporting material being employed. The figures given below were obtained using aluminium chloride as the halide and an evaporation temperature of 120 C.-

When ferro-aluminium containing 45.2% iron and consisting mainly of the compound AlsFez or the compounds AlaFe and AlzFe was brought into reaction with the aluminium chloride vapour at about 950 0., about 2.5 to 2.7 parts of aluminium chloride had to be passed through the ferroaluminium in order to distill one part of aluminium whereas at about 830 0. approximately 8 parts of A1013 had to be distilled for one part aluminium under the given conditions. The iron content of the distillate was of the order of 0.1% or less.

When ferro-aluminium containing 71.4% iron and mainly consisting of the compound AlFe was brought into reaction at about 1,000 C. the ratio of aluminium chloride to aluminium distilledwas about 2.5 to 2.7; at about 950 C. the ratio was between 3 and 8 dependent on and decreasing with the height of the ferro-aluminium column; and a temperature of 800 C. (with this aluminium bearing material) proved too low for satisfactory distillation.

Ezample IV Boron containing about 22% iron and 8% carbon was subjected to distillation according to the present invention, using borontrichloride as higher halide.

The distillation tower consisted of a steel pipe heavily calorized on the inside and lined with an alumina pipe. The lower end of the alumina pipe was fitted to a porous disc made from alumina on which the boron containing material was placed. The centre part of the steel pipe was part of a vacuum furnace heated electrically. The lower end of the steel pipe wasconnected in vacuumtight manner to the borontrichloride container, the connection including a vacuum valve. The top end of the steel pipe was connected in vacuum-tight manner to the borontrichloride condenser which itself 'was connected to a two stage oil vacuum pump. The pump permitted the whole system to be evacuated to a residual pressure of less than .02 mm. mercury.

The boron containing material was comminuted to a coarse powder standing on a 35 mesh sieve and passing through a 20 mesh sieve. It was then placed on the porous disc at the bottom of the alumina tube.

The borontrichloride container and condenser were first cooled to the temperature of liquid air and the whole system evacuated, and the pump kept going throughout the distillation.

The furnace and with it the boron containing material was then heated to about 1200 C. after which the borontrichloride container was warmed up to round about 100 C. Practically pure boron condensed inside the alumina tube in the form of thick crust at a temperature estimated at about 1000 C.

The amount of boron distilled under these conditions was many times more than the very small 12 amount which distilled without the borontrichloride stream.

I claim:

1. A process for distilling a normally non-volatile metal from material bearing the same, which comprises reacting at elevated temperatures the metal contained in the material with the vapour phase of a reacting halogen containing substance, brought to said phase previous to reaction, thereby to vapourize the said metal by a strongly endothermic reaction as its unstable lower halide, said temperatures, under the prevailing pressure, being below the temperature of direct evaporation of the said metal from the said material but high enough to enable said strongly endothermic reaction leading to the formation of the lower halide to proceed, and converting said lower halide into the metal and a stable halide by condensation of the metal to recover the metal therefrom.

2. A process for distilling a normally nonvolatile metal from material bearing the same, which comprises reacting at elevated temperatures the metal contained in the material with the vapour phase of a reacting halogen containing substance, brought to said phase previous to reaction, thereby to vapourize the said metal by a strongly endothermic reaction as its unstable lower halide, said temperatures, under the prevailing pressure, being below the temperature of direct evaporation of the said metal from the said material but high enough to enable said strongly endothermic reaction leading to the formation of the lower halide to proceed, and converting said lower halide into the metal and a stable halide by condensation of the metal brought about by cooling to recover the metal therefrom.

3. A process for distilling a normally nonvolatile metal i'rom material bearing the same, which comprises reacting at elevated temperatures the metal contained in the material with the vapour phase of a reacting halogen containing substance, brought to said phase previous to reaction, thereby to vapourize the said metal by a strongly endothermic reaction as its unstable lower halide, said temperatures, under the prevailing pressure, being below the temperature of direct evaporation of the said metal from the said material but high enough to enable said strongly endothermic reaction leading to the formation of the lower halide to proceed, and converting said lower halide into the metal and a stable halide by condensation of the metal brought about by absorption to recover the metal therefrom.

4. A process for distilling a normally nonvolatile metal from material bearing the same, which comprises reacting at elevated temperatures the metal contained in the material with the vapourphase of a higher halide of the metal, brought to said phase previous to reaction, thereby to vapourize the said metal by a strongly endo thermic reaction as its unstable lower halide, said temperatures, under the prevailing pressure, being below the temperature of direct evaporation of the said metal from the said material but high enough to enable said strongly endothermic reaction leading to the formation of the lower halide to proceed, and converting said lower halide into the metal and a stable halide by condensation of the metal brought about by cooling to recover the metal therefrom.

5. A process for distilling a normally nonvolatile metal from material bearing the same, which comprises reacting at elevated temperatures the metal contained in the material with the vapour phase of a halide of a volatile metal, brought to said phase previous to reaction, thereby to vapourize the said metal by a strongly endothermic reaction as its unstable lower halide, said temperatures, under the prevailing pressure, being below the temperature of direct evaporation of the said metal from the said material but high enough to enable said strongly endothermic reaction leading to the formation of the lower halide to proceed, and converting said lower halide into the metal and a stable halide by condensation of the metal brought about by cooling to recover the metal therefrom.

6. A process for distilling a normally nonvolatile" metal from material bearing the same, which comprises reacting at elevated temperatures the metal contained in the material with the vapour phase of a substance capable of reacting with the metal to form its higher halide, brought to said phase previous to reaction, thereby to vapourize the said metal by a strongly endothermic reaction as its unstable lower halide, said temperatures, under the prevailing pressure, being below the temperature of direct evaporation of the said metal from the said material but high enough to enable said strongly endothermic reaction leading to the formation of the lower halide to proceed, and converting said lower halide into the metal and a stable halide by condensation of the metal brought about by cooling to recover the metal therefrom.

7. A process for distilling a normally nonvolatile metal from material bearing the same, which comprises reacting at elevated temperatures the metal contained in the material with the vapour phase of a higher halide of the metal, brought to said phase previous to reaction, thereby to vapourize the said metal by a strongly endothermic reaction as its unstable lower halide, said temperatures, under the prevailing pressure, being below the temperature of direct evaporation of the said metal from the said material but high enough to enable said strongly endothermic reaction leading to the formation of the lower halide to proceed, and converting said lower halide into the metal and a stable halide by condensation of the metal brought about by absorption to recover the metal therefrom.

8. A process for distilling a normally nonvolatile metal from material bearing the same, which comprises reacting at elevated temperatures the metal contained ln the material with the vapour phase of a halide of a volatile metal, brought to said phase previous to reaction, thereby to vapourize the said metal by a strongly endothermic reaction as its unstable lower halide, said temperatures, under the prevailing pressure, being below the temperature of direct evaporation of the said metal from the said material but high enough to enable said strongly endothermic reaction leading to the formation of the lower halide to proceed, and converting said lower halide into the metal and a stable halide by condensation of the metal brought about by absorption to recover the metal therefrom.

9. A process for distilling a normally nonvolatile metal from material bearing the same, which comprises reacting at elevated temperatures the metal contained in the material with the vapour phase of a substance capable of reacting with the metal to form its higher halide, brought to said phase previous to reaction, thereby to vapourize the said metal by a strongly endothermic reaction as its unstable lower halide, said temperatures, under the prevailing pressure, being below the temperature of direct evaporation of the said metal from the said material but high enough to enable said strongly endothermic reaction leading to the formation of the lower halide to proceed, and converting said lower halide into the metal and a stable halide by condensation of the metal brought about by absorption to recover the metal therefrom.

10. A process for distilling a normally nonvolatile metal from material bearing the same, which comprises reacting at elevated temperatures and under partial vacuum the metal contained in the material with the vapour phase of a reacting halogen containing substance, brought to said phase previous to reaction, thereby to vapourize the said metal by a strongly endothermic reaction as its unstable lower halide, said temperatures, under said partial vacuum, being below the temperature of direct evaporation of the said metal from the said material but high enough to enable said strongly endothermic reaction leading to the formation of the lower halide to proceed, and converting said lower halide into the metal and a stable halide by condensation of the metal to recover the metal therefrom.

11. A process for distilling a normally nonvolatile metal from material bearing the same, which comprises reacting at elevated temperatures the metal contained in the material with an unsaturated vapour of a reacting halogen containing substance, said substance being vapourized previous to reaction, and said vapour under conditions of reaction having a small partial pressure in comparison with its saturation pressure at said temperatures, thereby to vapourize the said metal by a strongly endothermic reaction as its unstable lower halide, said temperatures, under the prevailing pressure, being below the temperature of direct evaporation of the said metal from said material but high enough to enable said strongly endothermic reaction leading to the formation of the lower halide to proceed, and recovering the metal from the lower halide containing vapour by condensation of the metal, thereby converting said vapour into the metal and a stable halide of the metal, said condensation beingbrought by cooling of the metal at a temperature higher than the condensation temperature of said stable halide.

PHILIPP GROSS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,184,705 Willmore Dec. 26, 1939 2,236,234 Hanak Mar. 25, 1 941 

